In deep water, the use of bottom-founded structures for oil well drilling and production operations is cost prohibitive due to the expense for fabrication and installation of such large structures. For water depths in excess of 1,000 feet, buoyant offshore structure moored to the sea floor can be used to perform drilling and production operations cost effectively.
As water depth exceeds 1,000 feet, the tension leg platform (TLP) concept can be introduced to perform oil drilling and production operations. A TLP consists of a buoyant offshore structure moored to fixed sea floor anchor points with vertical tension legs; also referred to as tendons. Drilling, producing and processing equipment as well as crew's quarters are contained in or on the buoyant offshore structure.
Tendon designs include both cable and tubular leg elements. U.S. Pat. No. 4,285,615, issued Aug. 25, 1981 to Frederick J. Radd discloses, "A mooring apparatus for a structure floating on a body of water, comprising: a corrosion resistant cable system, including a multi-strand cable, having voids between adjacent strands;". U.S. Pat. No. 4,226,555, issued Oct. 7, 1980 to Henry A. Bourne, Jr. discloses, "A mooring system for a tension leg platform, comprising: a tension leg, including a plurality of tubular leg elements having threaded connections between adjacent leg elements;".
The use of pre-tensioned vertical mooring elements prevents vertical motion but permits travel motion of the floating structure during the passage of waves. Pre-tensioning is accomplished by deballasting the buoyant offshore structure after the tendons are connected between the buoyant structure and fixed sea floor anchor bases.
Tendon inspection is necessary as both a maintenance expenditure and safety precaution. Tendon repair and replacement are both very expensive and laborious operations. Cracks and corrosion due to exposure to sea water decrease the failure load and working lifetime of the tendon. The desirability of minimizing tendon corrosion has been recognized in the art. Previously cited U.S. Pat. No. 4,285,615 discloses an invention for providing a corrosion resistant design for a tension leg cable which isolates the steel wire cable from the sea water environment.
The present invention provides a method and means for detecting structural deficiencies in a tubular tendon, increasing its buoyancy and extending its useful life.